A railroad vehicle is characterized in that because it travels by means of its iron wheels rolling over rail surfaces, its traveling resistance is smaller than that of an automobile. In particular, in a recent electric railroad vehicle, regenerative brake control is performed in which the main motor is operated as an electric power generator at the braking mode so that braking force is obtained and in which electric energy generated by the main motor at the braking mode is returned to the overhead line so as to be recycled as power running energy for other vehicles. It is known that an electric railroad vehicle equipped with a regenerative brake travels with approximately half energy consumption in comparison with an electric railroad vehicle that is not equipped with any regenerative brake. Therefore, an electric railroad vehicle equipped with a regenerative brake is an energy-saving one that effectively utilizes its characteristic that the traveling resistance is small.
Meanwhile, it is the present situation that in a local railroad or the like where the transportation density is low, meticulous passenger service is realized at low cost, by utilizing a diesel railcar that does not require any infrastructure such as an overhead line or a substation. However, because a diesel railcar has no means for regenerate energy for other vehicles through an overhead line or the like, the recycle of regenerative energy, which is performed by an electric railroad vehicle, has not been carried out. Accordingly, it has been considered that in order to realize energy-saving travel of a diesel railcar, there is no choice but to rely on the development of a high-mileage engine.
As a method of promoting energy saving even for such a diesel railcar, there has been proposed a hybrid diesel railcar in which an engine, an electric power generator, and an electric storage device are combined with one another (e.g., refer to Patent Document 1). In the hybrid diesel railcar, because a battery is provided, it is made possible that at the braking mode, the battery temporarily absorbs generative energy generated by the main motor, and the absorbed regenerative energy is recycled as part of energy required at the power running mode so that the energy saving can be realized.
In contrast, when the hybrid diesel railcar continues to descend a long distance with the regenerative brake, the battery is overcharged; thus, it is required to suppress the battery charging power through some sort of method. As a method for that, there is utilized a so-called engine brake in which the electric power generator is operated as a motor and the engine is utilized as a rotation friction load so that energy is intentionally consumed. In contrast, it is also made possible that as braking force for a vehicle, not a regenerative brake but a mechanical brake is utilized so that energy itself is suppressed and hence overcharging is prevented; however, when the time and trouble for the maintenance of a brake shoe, which wears away, is taken into consideration, it is desirable to refrain from utilizing the mechanical brake as much as possible; therefore, the usage of the engine brake has advantage.
The hybrid diesel railcar disclosed in Patent Document 1 is configured with an engine, an AC power generator that is driven by the engine so as to output AC power, a converter that converts AC power into DC power, an inverter that inverts DC power into AC power, a motor that drives a railroad vehicle, a speed reducer that reduces the rotation of the motor and transfers it to a wheel axle, an electric storage device that has a function of performing charging and discharging with DC power, a service-power-source inverter, a service-power-source transformer, and a control apparatus; however, the control method at the foregoing engine braking mode is not described in detail.